1. Field of the Invention
The present invention relates to an optical disc apparatus having a photo detector in which a detecting area is divided into four regions, and more particularly to an optical disc apparatus in which a crosstalk component in a focus error signal produced by a tracking error signal is reduced.
2. Description of the Related Art
In an apparatus which has a photo detector configured by four detecting areas arranged in a two by two matrix, and which has a function of writing information onto an optical disc such as a DVD or a CD, when the attaching position of the photo detector involves an error, a phenomenon that a tracking error component affects a focus error component occurs in a track jump in which a laser spot crosses lands and grooves. This phenomenon is called a T-F crosstalk. When a T-F crosstalk occurs, there arise problems in that defocusing is caused, and that focus-on is hardly attained. Therefore, it is required to attach a photo detector with high accuracy. However, it is often difficult to attain such high accuracy in manufacturing process of the apparatus.
Therefore, a conventional technique (referred to as a first conventional technique) has been proposed in which, even when a T-F crosstalk occurs, the level of the T-F crosstalk is lowered, whereby situations such as those where focus-on is hardly attained, and where defocusing easily occurs are less caused, so that the tolerance of attachment of a photo detector can be increased (a part of apparatuses which have been regarded as defective due to a large positional deviation of a photo detector can be regarded as nondefective so as to be useful). In this technique, a radial push-pull signal (tracking error signal) which is a difference signal between a result of an addition of two detecting areas that are positioned on one side with respect to the track axis, and that of an addition of two detecting areas that are positioned on the other side is multiplied with a tangential push-pull signal which is a difference signal between a result of an addition of two detecting areas that are positioned on one side with respect to an axis orthogonal to the track axis, and that of an addition of two detecting areas that are positioned on the other side. A signal which is obtained by multiplying a result of the multiplication by K is subtracted from a focus error signal, whereby the T-F crosstalk is reduced (for example, see JP-A-10-064080).
Also the following conventional technique (referred to as a second conventional technique) has been proposed. In this technique, the level of a tracking error signal is divided into plural level stages, and a track jump for level sampling is previously performed. In the track jump, for each of the divided levels of the tracking error signal, the level of the corresponding focus error signal (the level of a T-F crosstalk) is stored into a waveform memory. Namely, the level of the tracking error signal corresponds to that of the T-F crosstalk. In a track jump for the regular seeking procedure or the like, the level of the focus error signal is corrected by the level of the focus error signal (the level of the T-F crosstalk) which is previously stored in the waveform memory in correspondence with (is previously brought into correspondence with) the level of the tracking error signal when a laser spot crosses a track. Therefore, the corrected focus error signal is a signal in which the T-F crosstalk is eliminated (for example, see JP-A-2001-222827, specifically in paragraph [0026]).
In the first conventional technique, the signal which is obtained by multiplying the result of the multiplication of the tangential push-pull signal and the radial push-pull signal by K is always subtracted from the focus error signal. In some cases, level variations of the focus error signal in a track jump are produced by a T-F crosstalk, and, in other cases, such level variations are produced by another cause. In the case where level variations of the focus error signal are produced by a cause other than a T-F crosstalk, when a process of subtracting a signal which is obtained by multiplying the result of the multiplication of the tangential push-pull signal and the radial push-pull signal by K from the focus error signal is performed, therefore, the level change width of the focus error signal after the process is sometimes larger than that before the process. When such a situation arises, focus disturbances such as defocusing easily occur.
This problem is produced similarly in the second conventional technique in the following manner. In the case where level variations of the focus error signal are produced by a T-F crosstalk, the level of the focus error signal stored in the waveform memory indicates the level of the T-F crosstalk. By contrast, in the case where level variations of the focus error signal are produced by a cause different from a T-F crosstalk, the level of the focus error signal stored in the waveform memory shows variations which are not correlated with the period of the tracking error signal. When, even in this case, the focus error signal is corrected by the level stored in the waveform memory, the level change width of the focus error signal after the process is larger than that in the case where the correction is not performed. As a result, a situation arises in which focus disturbances such as defocusing more easily occur.